FIELD OF THE INVENTION
The invention relates to a device and method for the rapid digital/analog ("D/A") conversion of pulse width modulated ("PWM") signals, particularly of digitized analog output signals from a lambda probe in a motor vehicle that have been converted into PWM signals.
For cost reasons, modern microcontrollers are increasingly being used for digital signal processor applications in control systems. For such use, the analog measured variable is digitized using an analog/digital (A/D) converter, is modified by a program in the microcontroller (filtering, comparison with nominal variables, PID controller, etc.), and is converted subsequently into an analog signal again using a D/A converter. Although microcontrollers usually contain A/D converters for digitizing analog measured variables, a D/A converter is generally not integrated.
One D/A conversion option is to use an external D/A converter; this is the technically optimum solution, but also the most cost intensive. In a serial D/A converter having a clock frequency of, for example, 500 kHz with an accuracy of 10 bits, the conversion time arising is 2 ms in theory, but approximately 3 ms in practice. The conversion time is incorporated in the phase shift of the control loop as a delay time.
A further option is to convert the digital signal in the microprocessor into a signal having a fixed frequency and having a pulse duty factor that is proportional to the digital value, with subsequent external low pass filtering to remove the AC voltage components. This solution is much less expensive than a D/A converter and is, on many occasions, used for less demanding applications. For use in a control system, however, this solution has the disadvantage that the low pass filter required for filtering greatly delays the phase of the signal (group delay time). If the delay is within the control loop, then the delay contributes to the total phase delay for the control loop.
For example, in a third order low pass filter, the attenuation at a frequency of 100 Hz is -60 dB. In other words, the amplitude of a PWM signal at 5V is attenuated to 5 mV. Then, the group delay time is approximately 50 ms. If the PWM frequency is raised to 250 Hz and the low pass filter is matched accordingly, the group delay time would be reduced to 20 ms, which is roughly equivalent to the technical limit of the system.
Patent Abstracts of Japan Vol. 016, No. 055 (E-1165), Feb. 12, 1992 (Feb. 12, 1992)-&JP 03 254214 A (Fuji Electric Co Ltd), Nov. 13, 1991 (Nov. 13, 1991) disclose a device for the D/A conversion of PWM signals that has a flip-flop, two integrators, and a decoupling amplifier. One of the integrators integrates the PWM signal during a High phase and supplies the D/A converted voltage at the output at least during the subsequent Low phase. In the Low phase, the other capacitor is discharged in the meantime.
Patent Abstracts of Japan Vol. 006, No. 119 (E-116), Jul. 3, 1982 (Jul. 3, 1982)-&JP 57 048826 A (Japan Electronic Control Syst. Co. Ltd.), Mar. 20, 1982 (Mar. 20, 1982) disclose a device for the D/A conversion of PWM signals having a monostable multivibrator, an integrator, a storage capacitor, and a decoupling amplifier, the PWM signal being integrated during its Low phase.
German Patent No. DE 32 37 386 C2 discloses a digital/analog converter of the counter type for processing a pulse code modulated stereo sound signal, in which the alternately appearing digital data words for the left and right audio channels are serially converted into corresponding analog currents. The corresponding analog currents are converted in parallel in a respective integrator into voltage signals that subsequently pass through a respective low pass filter.